1. Field of the Invention
The present invention relates to apparatus for producing a scanning laser beam of constant linear velocity, where velocity is used in the vector sense of both magnitude and direction. Specifically, the invention relates to opto-mechanical apparatus which is useful for the rapid and accurate non-contacting measurement of object dimensions, hole dimensions, object positions and sheet thickness.
2. The Prior Art
For the accurate measurement of the diameter, position, or thickness of soft, delicate, hot or moving objects, noncontacting sensors must be used. Prior art devices of this character include capacitive gauges, eddy-current gauges, air gauges, gamma- and X-ray gauges and optical sensors. Only the optical and nuclear gauges can work (with sufficient sensitivity) at distances greater than a small fraction of an inch. The nuclear gauges permit large working distances; however, they are extremely expensive and susceptible to systematic errors due to slight variations in the chemical composition of the object being measured.
Optical sensors have advantages due to the nature of light itself. The principal advantages are:
1. They do not require direct mechanical contact between the sensor and the object to be measured.
2. The distance from the sensor to the object to be measured can be large.
3. The response time is limited to that of the photodetector and its electronics; and
4. Light variations are directly convertible to electrical signals.
5. The measurements are independent of the chemical composition of the object.
Optical sensors desireably should utilize scanning light beams of uniform linear velocity. Prior-art optical sensors employ various techniques in attempts to produce such a scanning light beam. For example, U.S. Pat. No. 3,533,701 discloses an optical gauge wherein a light beam is scanned mechanically by a reflector prism attached to a belt. This technique aims to convert a uniform rotation into a uniform linear velocity. Nevertheless, it possesses two serious disadvantages. Firstly, since it depends on an elaborate arrangement of wheels, bearings, belts, and linkages, the accuracy and reliability are questionable for serious industrial use. Secondly, since it is necessary to move relatively massive parts, the scan speed is slow and, therefore, the duration of measurement is necessarily long. The longer the duration of measurement, the greater is the susceptibility to erroneous readings because of object vibration.
An example of an optical attempt to produce a uniformly moving beam can be found in U.S. Pat. No. 3,765,774, which discloses an optical measuring apparatus, wherein a laser beam scanned in a parallel fashion is used to measure the diameter of objects. In this disclosure, a laser beam which is reflected from a uniformly rotating mirror is converted into a rotary scanned laser beam. A scanner lens is positioned with its optical axis in the plane of the rotary scanned light beam and converts the rotary scanning light beam into a parallel scanning light beam. The scanner lens is a singlet of conventional design. The rotating mirror is placed at the back focal point of the lens. The lens brings the laser beam to focus at its front focal plane. Therefore, the linearly scanned beam does not move with constant velocity since a conventional collimator does not convert uniform rotation into uniform linear motion.
While these prior-art techniques for producing a linearly scanned light beam are useful for some measurements, they cannot be used for the accurate control of many industrial operations. For example, in the high-speed extrusion of aluminium rod, it is desirable to get readings with accuracies of 0.0001 inch. The extruded rod moves in every direction as it comes out of the die so that the sensor must be capable of fast, accurate measurements over a large measurement of volume, i.e., several inches on a side, and with a short measurement duration. The measurement of sheet thickness in a rolling mill is an example where a high sensitivity sheet thickness with a large working distance, large measurement range, and short measurement duration is required.